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1) Levelling was slightly low so it's not sticking to the blue tape and instead sticking to the print head.

2) Axes are slipping.

#1 you can fix by just loosening the 4 screws a 1/4 turn.

#2 is trickier. You could mark all 12 pulleys and rods with a sharpie to see which are slipping. Or you could just tighten the hell out of them. Or do both. You should tighten the set screws so much that the steel shaft of the screwdriver is twisting. Alternatively you can replace the black set screws with the shiny ones that came seperate. The shiny ones have a sharper point and work better although I'm still using the black ones just fine. Make sure you get all 6 on each axis - the two hardest to get to on the short belt are the most likely to be slipping. Also add a drop of light oil to the 6 rods and also the 2 vertical Z rods and make sure you greased the Z screw with the green stuff. Light oil should be mineral oil with as few additives as possible (WD40 has some cleaning additives that should be avoided but it's probably okay).

More on how I level:

leveling here is defined as setting the Z height and also leveling. It's one procedure that does both at once.

Optionally heat up the nozzle to 180C because a cold nozzle shrinks and you will be setting the bed to the wrong height. Make sure tip of nozzle doesn't have any plastic on it or you may level to the wrong height. I usually prefer to level with a cold nozzle but if you want extra accuracy then use hot nozzle. If you have a heated bed that should also be warm for the same reason.

Home the z axis only. If you must home all 3 then you need to disable the steppers once it's done so you can move the print head by hand. Move the head as close as possible to each of the 4 screws in turn. Once at a screw tighten the screw and then slip a piece of paper between the nozzle and the print bed. Make sure the paper slides very freeley. Then loosen the screw until the paper gets slightly stuck. You want the paper to easily be able to slide in and out under the nozzle with one hand pushing the paper. If the paper gets stuck it's probably too tight under there. While levelling try rocking the bed across both opposite diagonals to see if one of the screws isn't touching the bed and to see if the bed is "hung up" or stuck on the threads of a screw instead of on the screw head.

Repeat this procedure for the other 3 screws. Then go back to the first screw and repeat on all 4 screws again. Then repeat on all 4 screws again. Then again. It may take you 20 minutes to do this the first time but the second time you do this it should take much less time because you are both better at it, faster at it, and because there isn't much to adjust the second time.

Note that the act of putting the screwdriver in the head can apply weight to the bed and change the height. You have to be careful to put as little weight as possible when testing with the paper.

If you leveled with a cold nozzle you are done. If you leveled with a hot nozzle you should then loosen the 4 screws 1/8 of a turn to compensate for the thickness of the paper.

Once done leveling rotate the z screw by hand to keep the nozzle off your bed. This makes it less likely to damage your bed surface and gives the nozzle room to leak.

On a new ultimaker repeat this procedure before every print (at least every hour) because the print bed can move/droop like a new guitar string. After many months the droop slows down.

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Thanks for quick response. I could try your recommendations only this weekend.

I replaced every black screw with steel and tighten them extra tight. I marked pulleys and rods to see which one was slipping and all of them were at the same position as before printing. Also, took out X and Y-motor to tighten pulleys on them. Applied oil on all rods.

It keeps printing erratically. Extruder doesn't follow the trajectory and jumps chaotically. I attached another video with spinning rods on right back side.

I tried to move extruder manually, when printer is not printing, and it feels pretty tight. I checked belts and they look fine. It feels like motors sometimes don't have enough power to turn rods accurately. They make this clicking sound after I stop printing.

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I disassembled extruder moving rods and checked all parts involved in smooth movement of the extruder. After I reassembled it back it worked fine. I was able to print my first ultimaker robot.

I can't say what was the reason for erratic printing before ( probably my poor assembly).

While printing, I've noticed that left shaft tends to slide out through back openning, so I had to put cover back on. It doesn't look like it affected printing but I'd like to know if there is a way to fix it.

Since my problem was resolved, this topic can be considered as solved. I hope it help someone with similar issue.

Anton

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While printing, I've noticed that left shaft tends to slide out through back openning, so I had to put cover back on. It doesn't look like it affected printing but I'd like to know if there is a way to fix it.

Um. Yeah. Put the cover back on.

You can put washers under the cover before putting screws through it. Or you can print these things:

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